Spectrum analyzer using multiple intermediate frequencies and multiple clock configurations for residual, spurious and image signal reduction

ABSTRACT

A method for spurious signal reduction when measuring a spectrum of a radio frequency signal over a frequency span. The radio frequency signal is converted to a sequence of intermediate frequency signals using a sequence of local oscillator signals each having a different frequency. The sequence of intermediate frequency signals are converted to a sequence of digitized intermediate frequency signal sample sets, each set having a frequency value and a signal energy value. The frequency span is divided into frequency sample bins. For each frequency sample bin, all frequency domain samples from the sequence of frequency domain sample sets having a frequency value within the frequency sample bin are associated with that sample bin. For each frequency sample bin, of the frequency domain samples associated with that frequency bin, the frequency domain sample having the signal energy value that is lowest is selected for a combined frequency domain sample set.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. Non-Provisionalpatent application Ser. No. 14/642,373 filed Mar. 9, 2015, now U.S. Pat.No. 10,145,872, which is a continuation application of U.S.Non-Provisional patent application Ser. No. 13/026,876 filed Feb. 14,2011, now U.S. Pat. No. 8,977,519, which claims priority to U.S.Provisional Patent Application Ser. No. 61/304,291 filed Feb. 12, 2010,all incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates generally to electronic test equipment,and more particularly, to radio frequency (RF) spectrum analyzers andmeasuring receivers.

BACKGROUND

The term “spectrum analyzer” refers to a device used to examine thespectral composition of a radio frequency (RF) input signal. A typicalspectrum analyzer will allow a user to select a frequency span definedby a span center frequency and a span width. The typical spectrumanalyzer will then divide the frequency span into segments (“bins”) andfor each frequency bin in the span, measure input signal energyassociated with frequencies in the bin. The result is typically shown ona visual display as a graph with the frequencies of the span on thehorizontal axis and the input signal energy per bin on the verticalaxis. The RF input signal may comprise one or more component signals atdifferent frequencies, each displayed as a peak on the vertical axis.

A digital spectrum analyzer typically consists of several sections,including an RF section, a local oscillator (LO) section, anintermediate frequency (IF) section, a digital processing section and adisplay. The RF section typically includes an attenuator to reduce theRF input signal amplitude, one or more stages of mixers to convert theRF input signal to an intermediate frequency (IF) signal using localoscillator signals from one or more local oscillators (LO) in the LOsection. The IF signal has several components. A desired component is afrequency shifted version of the input RF. The IF signal will also haveundesired components, including residual, spurious and image signals.The IF section typically has a system of filtering the IF signal toeliminate out-of-band signals, including IF signal components that arefrequency shifted replicas of RF signal components that were not in theselected span and including some of the unwanted residual, spurious andimage signals. The IF section also typically has an analog to digitalconverter for converting the analog IF signal to a set of digitized IFsignal samples. The digital processing section typically has hardware orsoftware for performing additional filtering, for performing FastFourier Transforms (FFT) of the time domain set of digitized IF signalsamples to a frequency domain set of digitized IF signal samples and formaking various measurements of the digital time and frequency domainsets of the IF signal samples.

Spectrum analyzers generate undesired residual, spurious and imagesignals. Residual signals are false signals that are displayed with noinput into the spectrum analyzer, and are typically generated from theelectronic circuitry of the spectrum analyzer itself. Spurious signals(“spurs”) are false signal products that result when an input signal isapplied. Image signals are the undesired one of a summed frequencysignal and a difference frequency signal, both of which are generatedwhen mixing the RF input signal with an LO signal.

To minimize the introduction of unwanted residual signals, a spectrumanalyzer typically isolates the LO, RF, IF and digital processingsections using shielding, which adds considerable weight and cost.Spectrum analyzers often use a Yttrium Iron Garnet (YIG) LO, whichrequires several watts of power and is expensive, but provides a cleansignal which adds minimal phase noise to the input signal.

Measuring receivers are used to measure precise relative signalamplitude measurements over a wide dynamic range, measure peak andaverage modulation characteristics and apply filters to the IF analogand digital signals. Frequency modulation and amplitude modulationcharacteristics may be measured. Precise amplitude steps may be measuredas well. Measuring receivers are typically separate devices fromspectrum analyzers, in spite of sharing many functional blocks.

Many modern spectrum analyzers use the Fast Fourier Transform (FFT)technique to convert time-domain signal data into frequency-domainsignal data. Processing high-resolution FFTs quickly requires a powerfulprocessor.

Handheld spectrum analyzers contain less expensive, less accuratecomponents, have less shielding and consume less power than atraditional rack-mount spectrum analyzer. They typically have a lowresolution display and buttons for a user interface. They are generallynot capable of processing automated commands and are of minimalusefulness in a lab setting. They generally have slower processors whichare not capable of quickly processing very large FFTs. With lessaccurate components, less powerful processors and less shielding,handheld spectrum analyzers do a poor job of reducing residual, spuriousand image signals and have poorer overall results than a larger, highquality spectrum analyzer.

RF cables are often used to connect spectrum analyzers to a signal beingmeasured and can be a major source of measurement inaccuracies. RFcables typically have unknown, frequency-dependent losses which changeas the cable is bent or twisted. It is often not very convenient toplace a large spectrum analyzer near the source of an RF signal ororient it in such a way to minimize bending of the RF cables.

Owning a modern spectrum analyzer with good specifications is currentlycost prohibitive for many students, inventors, and amateur radioenthusiasts.

What is needed is an ultra-low-cost, low-power, lightweight, portablespectrum analyzer similar in size and weight to a traditional RF powersensor; a spectrum analyzer that can be connected close to the source ofa signal being measured without long intervening RF probe cables yetcarries the signal processing power of the modern personal computer.

SUMMARY AND ADVANTAGES

An embodiment of a spectrum analyzer is disclosed herein for measuringan RF signal over a selected frequency span, the spectrum analyzerconfigured to use multiple Local Oscillator (LO) signals to generatemultiple Intermediate Frequencies (IFs) for residual, spurious and imagesignal reduction. The disclosed spectrum analyzer has both a primary IFpath and a secondary IF path, each configured to provide band passfiltering of the multiple IF signals. A master clock synthesizer isconfigured to reduce residual noise by providing a master clock signaland a second stage Local Oscillator (LO) signal, both from a singleVoltage Controlled Oscillator (VCO). The spectrum analyzer has amicrocontroller configured to change the frequency of the master clocksignal and the second stage LO signal if the center frequency of theselected span is sufficiently close to a known spurious signal using thecurrent second stage LO signal. The disclosed spectrum analyzer performsspectrum analysis on DC-coupled RF signals from 1 Hz to 4.4 GHz. Thedisclosed spectrum analyzer embodiment is USB bus-powered.

A method is disclosed for the selection of variable LO, clock, and IFfrequencies to effectively mask out image, spurious and residual signalsacross several ranges of input frequencies. A method is disclosed forreducing the effects of internally generated residual signals. A methodis disclosed for providing rapid frequency sweeps of large spans withoutthe use of a sweep oscillator.

Additional advantages of the invention will be set forth in part in thedescription which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. Theadvantages of the invention may be realized and attained by means of theinstrumentalities and combinations particularly pointed out in theappended claims. Further benefits and advantages of the embodiments ofthe invention will become apparent from consideration of the followingdetailed description given with reference to the accompanying drawings,which specify and show preferred embodiments of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of this specification, illustrate one or more embodiments of thepresent invention and, together with the detailed description, serve toexplain the principles and implementations of the invention.

FIG. 1 is a block diagram of the spectrum analyzer in accordance with anexemplary embodiment of the present invention.

FIG. 2 shows a flow chart of a method used by the spectrum analyzer ofFIG. 1 for measuring the RF signal.

FIG. 3 shows a flow chart of a method for a spectrum analyzer to changeclocking configuration to eliminate spurious signals when measuring anRF signal over a selected frequency span with a center frequency.

FIG. 4 shows a flow chart of a method for changing clock configuration.

REFERENCE NUMBERS USED IN DRAWINGS

Turning now descriptively to the drawings, in which similar referencecharacters denote similar elements throughout the several views, thefigures illustrate the spectrum analyzer of the present invention. Withregard to the reference numerals used, the following numbering is usedthroughout the various drawing figures:

Reference Numbers Part References 10 spectrum analyzer 11 RF input 12input attenuator 13 first RF path switch 14 high band mixer 16 widebandsynthesizer 17 low band divider 18 low band mixer 19 second RF pathswitch 20 first IF path switch 21 secondary IF filter 22 primary IFimpedance matching network 23 primary IF filter 24 wideband RMS powerdetector 25 second IF path switch 26 IF-to-Bits circuit IC 29 masterclock synthesizer 30 RF/IF section 31 personal computer PC 32 display 33microcontroller 34 memory 35 control line 36 Analog to Digital ConvertorADC 37 second stage mixer 38 mixer bypass 39 mixer bypass switch 40 dataport 41 processor 42 pc memory 45 clock divider 47 clock line 49 secondstage LO divider 51 reference signal input 53 reference divider 55master phase comparator 57 master VCO 59 master VCO divider 61 LOreference signal input 65 LO phase comparator 67 LO VCO 69 LO VCOcounter 71 high band divider

DETAILED DESCRIPTION

Before beginning a detailed description of the subject invention,mention of the following is in order. When appropriate, like referencematerials and characters are used to designate identical, corresponding,or similar components in differing figure drawings. The figure drawingsassociated with this disclosure typically are not drawn with dimensionalaccuracy to scale, i.e., such drawings have been drafted with a focus onclarity of viewing and understanding rather than dimensional accuracy.

In the interest of clarity, not all of the routine features of theimplementations described herein are shown and described. It will, ofcourse, be appreciated that in the development of any such actualimplementation, numerous implementation-specific decisions must be madein order to achieve the developer's specific goals, such as compliancewith application- and business-related constraints, and that thesespecific goals will vary from one implementation to another and from onedeveloper to another. Moreover, it will be appreciated that such adevelopment effort might be complex and time-consuming, but wouldnevertheless be a routine undertaking of engineering for those ofordinary skill in the art having the benefit of this disclosure.

FIG. 1 shows an embodiment of a spectrum analyzer 10 configured togenerate multiple Intermediate Frequencies (IF) for residual, spuriousand image signal reduction. The spectrum analyzer 10 is configured toexamine the spectral composition of a radio frequency (RF) input signal.The spectrum analyzer 10 is configured to allow a user to select afrequency span by selecting a span center frequency and a span width.The spectrum analyzer 10 is configured to divide the frequency span intosegments (“bins”) and for each frequency bin in the span, measure inputsignal energy associated with frequencies in the bin. The spectrumanalyzer 10 is comprised of an integrated RF/IF section 30, a personalcomputer (PC) 31 configured for digital signal processing and a display32. The RF/IF section 30 is configured to convert the RF signal to afirst set of digitized IF signal samples using a first Local Oscillator(LO) signal that has a first LO frequency and to convert the RF signalto a second set of digitized IF signal samples using a second LocalOscillator (LO) signal that has a second LO frequency. The PC 31 isconfigured to convert the first and second set of digitized IF signalsamples to first and second sets of frequency domain samples. The PC 31is further configured to combine the first and second set of frequencydomain samples into a combined set of frequency domain samples byselecting, for each frequency sample bin, the lower valued frequencydomain sample of the first and second IF frequency domain samplesassociated with that frequency sample bin. The display 32 is configuredto visually present the combined frequency domain samples.

The RF/IF section 30 has master clock synthesizer 29 configured toprovide clocking to the various components of the RF/IF section usingmethods that reduce or eliminate unwanted residual and spurious signals.The RF/IF section 30 includes an RF/IF microcontroller 33 configured toexecute various procedures to control and coordinate the actions of thevarious RF/IF section 30 components over control lines 35 and an RF/IFmemory 34 configured to store the instructions for the various controlprocedures.

An RF portion of the RF/IF section 30 includes an RF input 11, an inputattenuator 12, a first RF path switch 13, a high band mixer 14, a lowband mixer 18, a second RF path switch 19, a wideband synthesizer 16,and a low band divider 17. A high band RF path is defined as a signalpath from the first RF path switch 13 through the high band mixer 14 tothe second RF path switch 19. A low band RF path is defined as a signalpath from the first RF path switch 13 through the low band mixer 18 tothe second RF path switch 19. An RF signal applied to the RF input 11 isrouted to the input attenuator, which in the exemplary embodiment is aCMOS digital step attenuator. The attenuated RF signal is then routed tothe first RF path switch 13, selecting the high band RF path leading tothe high band mixer 14 or the low band RF path leading to the low bandmixer 18. A high band local oscillator (LO) signal for the high bandmixer 14 is generated by the wideband synthesizer 16. A low band LOsignal for the low band mixer 18 comes from the low band divider 17which is driven by the high band LO signal generated by the widebandsynthesizer 16. When the first and second RF path switches are selectedfor the high band RF path, the attenuated RF signal is mixed with thehigh band LO signal in the high band mixer 14 to produce an IF signal.When the first and second RF path switches are selected for the low bandRF path, the attenuated RF signal is mixed with the low band LO signalin the low band mixer 18 to produce an IF signal. All IF signalsgenerated by either the high band mixer 14 or the low band mixer 18 areconsidered first stage IF signals. Two different RF paths are providedbecause a practical RF mixer will not perform well at both high and lowfrequencies. The low band mixer 18 is selected to have good performanceat low frequencies and the high band mixer 14 is selected to have goodperformance at high frequencies. In the exemplary embodiment, the lowband mixer 18 is a Mini-Circuits ADE-1ASK and the high band mixer is aMini-Circuits ADE-42MH. In the exemplary embodiment, the low band RFpath is used for RF signals with frequencies equal to or less than 150MHz and the high band RF path is used for RF signals with frequenciesgreater than 150 MHz.

The RF/IF section 30 is configured to generate each LO signal with afrequency selected such that when the LO signal is mixed with the RFsignal, the IF signal generated is the RF signal frequency shifted to aprimary IF or a secondary IF. That is, the IF signal will have energy atthe primary IF in proportion to energy at the center span frequency inthe RF signal and, each IF frequency will have energy in the sameproportion to energy in an RF frequency where the difference between theIF frequency and the primary IF is the same as the difference betweenthe RF frequency and the center span frequency of the RF signal. In theexemplary embodiment, the primary IF is 10.7 MHz and the secondary IF is2.9 MHz.

The RF/IF section 30 is configured to sequentially convert the RF signalto a first IF signal using a first LO signal with a first LO frequency,then convert the RF signal to a second IF signal using a second LOsignal with a second LO frequency. In some instances, both first andsecond IF signals are generated sequentially by the high band mixer 14,one after the other. In other instances, one of the IF signals isgenerated by the high band mixer 14 while the other IF signal isgenerated by the low band mixer 18. In yet other instances, both firstand second IF signals are generated sequentially by the low band mixer18, one after the other.

The low band mixer 18 is placed in the low band RF signal path with IFand RF ports reversed, allowing frequencies down to DC to beup-converted, or higher frequencies to be down-converted, withoutchanging hardware configuration.

The RF/IF section 30 includes an IF filtering portion with a first IFpath switch 20, a secondary IF filter 21, a primary IF impedancematching network 22, a primary IF filter 23, a wideband RMS powerdetector 24 and a second IF path switch 25. A primary IF path is definedas a signal path from the first IF path switch 20 through the primary IFfilter 23 to the second IF path switch 25. A secondary IF path isdefined as a signal path from the first IF path switch 20 through thesecondary IF filter 21 to the second IF path switch 25. The secondary IFfilter 21 has center frequency selected to match the secondary IF. Theprimary IF impedance matching network 22 and primary IF filter 23 havecenter frequencies selected to match the primary IF. In the exemplaryembodiment, the secondary IF filter 21 has a center frequency of 2.9 MHzand the primary IF impedance matching network 22 and primary IF filter23 have center frequencies of 10.7 MHz. The bandwidth of the secondaryIF filter 21 and the primary IF filter 23 are selected to match thebandwidth of the ADC 36. The bandwidth of the primary IF impedancematching network 22 is selected to facilitate large span measurementsusing the RMS power detector 24, which will be described in greaterdetail below. In the exemplary embodiment, the secondary IF filter 21and the primary IF filter 23 have bandwidths of 280 kHz. The bandwidthof the primary IF impedance matching network 22 is 5 MHz.

The RF/IF section 30 is configured to sequentially convert the RF signalto a first IF signal using a first LO signal with a first LO frequency,then convert the RF signal to a second IF signal using a second LOsignal with a second LO frequency. In some instances, both first andsecond IF signals pass through the primary IF path, one after the other.In other instances, one of the IF signals passes through the primary IFpath while the other IF signal passes through the secondary IF path. Inyet other instances, both first and second IF signals pass through thesecondary IF path, one after the other.

The RF/IF section 30 includes a second stage IF mixing portion whichincludes a second stage mixer 37, a mixer bypass 38, and a mixer bypassswitch 39. The second stage IF mixing portion is followed by an Analogto Digital Converter (ADC) 36. If the second IF path switch 25 is set toshunt an IF signal from the primary IF path, then the mixer bypassswitch 39 is configured to shunt the IF signal to the second stage mixer37. The second stage mixer 37 is configured to convert the first stage,primary IF signal into a second stage IF signal using a second stage LOsignal with a second stage LO. The frequency of the second stage LO isselected to make center frequency of the second stage IF signal matchthe center frequency of the ADC 36. If the second IF path switch 25 isconfigured to shunt an IF signal from the secondary IF path, then themixer bypass switch 39 is set to shunt the IF signal through the mixerbypass 38 and straight to the ADC 36. The ADC 36 is configured toconvert either the second stage IF signal or the secondary IF signal,depending on the state of the mixer bypass switch 39, into a set ofdigitized IF signal samples comprising interleaved in-phase andquadrature-phase (I/Q) data samples. The ADC 36 sends the set ofdigitized IF signal samples as a serial digital signal, viamicrocontroller 33 and a data port 40, out of the RF/IF section 30 tothe PC 31. In the exemplary embodiment, the data port 40 conforms to oneof the Universal Serial Bus standards.

In the exemplary embodiment, an “IF-to-bits” integrated circuit (IC) 26,the Analog Devices AD9864, is used to provide the second stage mixer 37,the mixer bypass 38, the mixer bypass switch 39 and the ADC 36. In theAD9864, the center frequency of the ADC 36 is ⅛ of the frequency of theclock signal provided to the ADC 36.

The RF/IF section 30 having two IF paths can produce some distinctadvantages. In the exemplary embodiment, The 2.9 MHz secondary IF pathyields a flat response and is 10 dB more sensitive than the 10.7 MHzprimary IF path, but has much larger spurious signals from poorlyfiltered out-of-band signals. The 10.7 MHz primary IF path, with itsadditional filtering and mixing stage, is less sensitive but providesbetter out-of-band spurious rejection. Measurements are further enhancedby setting one local oscillator signal to yield an IF signal with the2.9 MHz secondary IF and using the 2.9 MHz secondary IF path whilesetting the second local oscillator signal to yield an IF signal withthe 10.7 MHz primary IF and using the 10.7 MHz primary IF path. Thedisplayed average noise level benefits from the sensitivity of the 2.9MHz secondary IF path while the spurious signals are by and large maskedout by combining the digitized 2.9 MHz IF signal with the 10.7 MHz IFsignal in the masking process.

The PC 31 has a processor 41 and a PC memory 42. The processor 41 isconfigured to execute digital signal processing (DSP) methods forprocessing the sets of digitized IF signal samples received from theRF/IF section 30. Such methods include performing FFTs on the first andsecond set of digitized IF signal samples to transform them into firstand second sets of frequency domain samples; and combining first andsecond sets of frequency domain samples into a composite set offrequency domain samples, masking out unwanted images and spurioussignals in the process. The PC memory 42 is configured to store theinstructions for the DSP methods. The PC memory 42 may also beconfigured to store the combined frequency signals. The PC memory 42 maybe volatile or non-volatile memory.

The RF/IF path switches (the first RF path switch 13, the second RF pathswitch 19, the first IF path switch 20, the second IF path switch 25 andthe mixer bypass switch 39) are all controlled by the RF/IFmicrocontroller 33 through control lines 35. In the exemplaryembodiment, the RF/IF path switches are transistor switches capable ofsub-millisecond switching.

The RF/IF section 30 has a master clock synthesizer 29 configured togenerate the second stage LO signal and a master clock signal. Themaster clock synthesizer 29 comprises an internal integer-N Phase LockLoop (PLL) that includes a reference signal input 51, a referencedivider 53, a master phase comparator 55, a master Voltage ControlledOscillator (VCO) 57, and a master VCO divider 59. The PLL is configuredto convert a frequency reference signal to a master VCO signal. Themaster VCO 57 generates the master VCO signal at a frequency controlledby a voltage supplied by the master phase comparator 55. The referencedivider 53 divides the frequency of the reference signal F_(ref) by areference divisor M, resulting in a signal to the master phasecomparator 55 with a frequency 1/M of the reference frequency. Themaster VCO divider 59 divides the frequency of the master VCO signal(F_(MVS)) by a master VCO divisor N_(MV), resulting in a signal to thephase comparator with a frequency 1/N_(MV) of the master VCO signalfrequency. As such, the PLL will lock the master VCO signal frequencyF_(MVS) equal to the reference signal F_(ref) times the referencedivisor M divided by the master VCO divisor N_(MV), as stated by theequation: F_(MVS)=F_(ref)×M N_(MV).

The master clock synthesizer 29 has a second stage LO divider 49configured to divide the master VCO signal by a second stage LO divisorinteger N_(LO2) to produce the second stage LO signal at a second stageLO frequency F_(LO2) that is 1/N_(LO2) of the master VCO signalfrequency. The master clock synthesizer 29 has a second stage LO signaloutput configured to send the second stage LO signal to the second stagemixer 37.

The master clock synthesizer 29 has a clock divider 45 configured todivide the master VCO signal by a clock divisor integer N_(CLK) toproduce a master clock signal at a master clock signal frequency F_(CLK)that is 1/N_(CLK) of the master VCO signal frequency. The master clocksynthesizer 29 has clock outputs configured to send the master clocksignal to the ADC 36 and also to the RF/IF microcontroller 33 via aclock line 47.

Providing both the clocking to the ADC 36 and the LO signal to thesecond stage mixer 37 from the same VCO reduces the amount of noiseintroduced to the IF signals. Additionally, all the other components inthe RF/IF section 30 that require clocking either receive the masterclock signal directly from the master clock synthesizer 29 or from theRF/IF microcontroller 33, which may divide down the frequency for othercomponents. Thus all major sources of clock noise are at substantiallythe same frequency or fractions thereof, which makes mitigating clockinduced noise a simpler task.

The values of the master clock synthesizer 29 divisors (the referencedivisor M, the master VCO divisor N_(MV), the clock divisor integerN_(CLK), and the second stage LO divisor integer N_(LO2)) are closelyinterrelated and should be selected carefully. There are severalconsiderations. In the exemplary embodiment, the master clocksynthesizer 29 is a Texas Instruments CDCE906. Values must be found sothat the master VCO 57 operates at frequencies in its allowable range,which for the CDCE906 is between 100 MHz and 200 MHz. Values must befound so that the resulting second stage IF signal after second stage LOmixing has a center frequency near to the center frequency of the ADC36, which for the AD9864 is one-eighth of the frequency of the clocksupplied to the ADC 36. Better control of phase noise is achieved whereN_(MV) is kept to a minimum and values selected that have been found notto produce overlapping spurious signals. In the exemplary embodiment, areference signal with a frequency of 10 MHZ is used.

In the exemplary embodiment, three configurations of the master clocksynthesizer 29 are used, as shown in Table 1:

TABLE 1 Config. M N_(MV) N_(CLK) N_(LO2) F_(MVS) F_(CLK) F_(LO2) 1 14 16 18 140 23⅓ 7 7/9 MHz MHz MHz 2 27 2 6 10 135 22½ 13.5 MHz MHz MHz 3327 2 7 21 163.5 23 6/17 7.784 MHz MHZ MHz

The values in configurations 1-3 were selected for their non-overlappingresidual and spurious signals as well as their low divisors. The thirdconfiguration is needed to avoid residual signals from the master clocksignal frequencies in the first two configurations, at multiples of 630MHz. The third configuration has spurious signals at frequencies thatare dissimilar to the other configurations.

If self-generated residual or spurious signals occur at the RF frequencybeing measured, the clock frequency is automatically changed to reduceinterference. Changing clock frequencies needs to occur quickly andseamlessly. To accomplish this, the RF/IF microcontroller 33 switchesthe clocking between measurements. The RF/IF microcontroller 33 firstswitches to an internal resistor-capacitor (RC) clock, then writes thenew clock settings to the master clock synthesizer 29, and finallyswitches back to the master clock signal when it is stable at the newmaster clock frequency. This is transparent to the user.

The wideband synthesizer 16 comprises a fractional-N Phase Lock Loop(PLL) that includes an LO reference signal input 61, an LO phasecomparator 65, an LO Voltage Controlled Oscillator (VCO) 67, and an LOVCO counter 69. The PLL is configured to convert a frequency referencesignal to an LO VCO signal. The LO VCO 67 generates the LO VCO signal ata frequency controlled by a voltage supplied by the LO phase comparator65. The reference signal F_(ref) is fed to the LO phase comparator 65.The LO VCO counter 69 alters the LO VCO signal frequency (F_(LVS)) by amultiple of an LO counter factor (CF_(LO)), which is equal to thequantity of an integer A, plus a numerator B divided by denominator C,the quantity divided by 2 to the power of a post-scalar factor P. Assuch, the PLL will lock the LO VCO signal frequency F_(LVS) equal to thereference signal F_(ref) times the LO counter factor (CF_(LO)), asstated by the equation: F_(LVS)=F_(ref)×(A+B/C)/2^(P). In the exemplaryembodiment, the wideband synthesizer 16 is an Analog Devices ADF4350.For the ADF4350 integer A can have values of 220-440 inclusive.Numerator B is forced to be less than 900, typically much less.Denominator C can be equal to or greater than 2 and less than 900, mustbe a prime number, and is typically less than 100. The post-scalarfactor P can be 0-4 inclusive, or 4-10 inclusive when using the ADF4007for the low band divider 17.

In the exemplary embodiment, reference signal F_(ref) at 10 MHz isconverted to an LO VCO signal at an LO VCO frequency F_(LVS) between 2.2GHz and 4.4 GHz, which is then divided in a high band divider 71 by 1,2, 4, 8, or 16 internally to produce the high band LO signal for thehigh band mixer 14. The high band LO signal is further dividedexternally by 8, 16, 32, or 64 in the low band divider 17 to produce thelow band LO signal for the low band mixer 18. In the exemplaryembodiment, the low band divider 17 is an Analog Devices ADF4007.

Fractional-N spur signals are injected into the IF signal by thewideband synthesizer 16 as a function of denominator C and post-scalarfactor P. As discussed above, image and spur rejection is achieved bymixing the RF signal with two distinct LO signals at differentfrequencies to generate two IF signals, combining the frequency-domainsamples of the two IF signals, then for each frequency bin in theselected span, accepting only the lower valued frequency domain sampleassociated with that frequency sample bin, thereby masking out unwantedresidual, spurious and image signals since each such unwanted signaloccurs at a different frequency in each of the two IF signals. For mostpairs of LO signals, values of the denominator C used to generate themwill be such that the fractional-N spur signals do not occur at the samefrequency and the spurs will be masked out. However, it is possible thatvalues of denominator C can be selected such that some of thefractional-N spurs occur at the same frequency and will not be maskedout. To ensure that fractional-N spurs from the LOs are masked out,values for the denominator C must be prime, but different, integers forthe two selected frequencies. The two distinct values of C each must bethe minimal values for which an LO frequency exists that is sufficientlyclose to the ideal LO frequency for the desired measurement. In theexemplary embodiment, an LO frequency within 5% of the center frequencyof the span under test+5 kHz is considered sufficiently close.

Additionally, the ADC 36 has a residual signal which needs to beeliminated. Integer A, numerator B, denominator C and post-scalar P areselected such that the LO spurs and the ADC 36 residual signals are notoverlapping. Thus any image, residual, and spurious signal do not lineup at the same frequency in an IF signal generated at one LO frequencyas they will in an IF signal generated at another LO frequency and arecanceled during the masking process.

In the exemplary embodiment, twelve distinct combinations of clocks andmixing are available for each RF frequency span examined. The LO signalcan be injected with a frequency on the high side of the RF signal or onthe low side, the resulting IF can be 2.9 MHz or 10.7 MHz, and one ofthree clock configurations can be selected. This allows substantialflexibility when reducing image, spur and residual signals. Thisflexibility in system clocks also reduces the need for shielding sincethe spectrum analyzer 10 can automatically configure clocking to avoidself-generated noise. This reduces overall size and weight, allowingconstruction of a smaller, more portable device. Increased portabilitywill in many cases eliminate the need for an RF cable between the RF/IFsection 30 and the source of RF signal being measured, yielding aspectrum analyzer that can be connected like an RF power sensor andbenefit from the increased accuracy from the lack of RF cables.

These features of the spectrum analyzer 10 allow RF and IF sections ofthe spectrum analyzer 10 to be integrated more closely and with lessshielding than in previous designs, providing for a more compact andlower cost device. However, one of skill in the art will realize thatthe various innovative features described herein may be practicedwithout integrating the RF and IF sections and eliminating shielding.

For very low RF frequencies, near or below the operating frequency ofthe low band mixer 18 or the low band divider 17, image suppressioncannot be achieved by mixing the RF signal with LO signals withfrequencies on the high side and low side of the RF signal. Instead, twolocal oscillator signals are used with LO signals with frequencies onthe high side of the RF signal separated only by tens of kilohertz. Theaforementioned masking technique is used to reject the image, since theimage will shift the opposite direction than the desired signal.

To achieve full span sweeps of 4.25 GHz in just a few seconds, the ADC36 cannot be utilized due to its narrow bandwidth. Therefore, a widebandRMS power detector 24 monitors the output of the primary IF impedancematching network 22. The bandwidth of ˜5 MHz allows stepping of the LOby 3 MHz across the selected span and taking a wide IF power levelreading at each step. By assuming the RF input signal will at each stepproduce a high-side IF signal with peaks at frequencies that are a sumof the RF and LO frequencies and a low-side IF signal with peaks atfrequencies that are a difference of the RF and LO frequencies, thespectrum analyzer 10 can mask out any low-side IF signal peak that doesnot have a corresponding high-side IF signal peak, then take a series ofmore accurate power readings at frequencies near the low side IF signalpeaks using the ADC 36. Thusly, a full span sweep can be achieved withfull amplitude accuracy in a few seconds.

FIG. 2 shows a flow chart of a method used by the spectrum analyzer 10of FIG. 1 for measuring the RF signal. Step 100 is selecting a frequencyspan. This step may be performed manually by a user manipulatingcontrols or by commands from an automated script. Step 102 is checkingwhether the clocking configuration needs to be changed. This is a checkfor known spurious signals generated by clocking within the spectrumanalyzer that may be near to the center frequency of the span. FIG. 3discusses this step is greater detail.

Step 104 is selecting an intermediate frequency (IF) for the firstsampling from one of a primary IF and secondary IF. To obtain ameasurement of the selected frequency span, at least two samplings areperformed. If the width of the span is larger than the bandwidth of theADC 36, then the span is divided into segments each equal to the widthof the ADC 36 bandwidth and two samplings are performed for eachsegment. Step 104 also includes selecting a mixer for a first sampling.Either the high band mixer 14 or low band mixer 18 may be selected. Inthe exemplary embodiment, the high band mixer 14 is selected in allcases except where the center frequency of the selected frequency spanis less than 150 MHz. Once IF and mixer have been selected for the firstsampling, the RF/IF path switches are set to create a first signal paththrough the mixer for the first sampling and an IF path associated withthe IF for the first sampling. Step 106 is setting a first stage localoscillator (LO) signal to a first LO frequency. This is based on theselected span and the IF selected for the first sampling. The LOfrequency must be such that its sum or difference with the centerfrequency of the selected span equals the selected IF. Step 108 istaking a first set of digitized IF samples with the ADC 36.

Step 110 is selecting a mixer for the second sampling. Once again,either the high band mixer 14 or the low band mixer 18 may be selected,regardless of the selection for the first sampling. Step 110 alsoincludes selecting an intermediate frequency (IF) for the secondsampling. Once again, either the primary IF or the secondary IF may beselected, regardless of the selection that was made for the firstsampling. Once IF and mixer have been selected for the second sampling,the RF/IF path switches are set to create a second signal path throughthe mixer for the second sampling and an IF path associated with the IFfor the second sampling. Step 112 is setting the first stage localoscillator (LO) to a second LO frequency. This is based on the selectedspan and IF for the second sampling. The LO frequency must be such thatits sum or difference with the center frequency of the selected spanequals the selected IF. Step 114 is taking a second set of digitized IFsamples.

Step 116 is transforming the first and second sets of digitized IFsamples to a first and a second set of frequency domain samples. This isdone with an FFT. Step 118 is combining the first and second set offrequency domain samples into a combined set of frequency domainsamples. The combining is done by looking at the samples associated withthe same frequency sample bin in both sample sets and selectingwhichever has a lower value frequency domain sample. The higher valuefrequency sample is discarded. In this way, unwanted signals are maskedout, as they will only have peak values in one of the two sample sets.

Step 120 is displaying the combined set of frequency domain samples. Inother embodiments, the combined set is stored. In yet other embodiments,the combined set is displayed and stored.

FIG. 3 shows a flow chart of a method for a spectrum analyzer to changeclocking configuration to eliminate spurious signals when measuring anRF signal over a selected frequency span with a center frequency. Step150 is checking the selected frequency span width and if the selectedspan is greater than 100 KHz wide then Step 164 is performed forchanging to a first clock configuration (see Table 1 for clockconfigurations) and the method terminates. If the selected span is notgreater than 100 KHz wide, then step 152 is performed.

Step 152 is checking if the center frequency of the selected span isnear a first known spurious signal or integer multiples thereof and ifso, Step 154 is performed for changing to a second clock configurationand the method terminates. In this method, the center frequency of theselected span is considered near a spurious signal if it is within halfof the ADC 36 bandwidth. In the exemplary embodiment, half of the ADC 36bandwidth is about 100 KHz. If the center frequency of the selected spanis not near the first known spurious signal or integer multiplesthereof, then step 156 is performed.

Step 156 is checking if the center frequency of the selected span isnear a second known spurious signal or integer multiples thereof and ifso, Step 158 is performed for changing to a third clock configurationand the method terminates. If the center frequency of the selected spanis not near the second known spurious signal or integer multiplesthereof, then step 160 is performed.

Steps 160 and 162 are repeats of steps 152 and 156 for an Nth knownspurious signal or integer multiples thereof. In the exemplaryembodiment, only two spurious signals are known: 630 MHz (and integermultiples thereof) and 23⅓ MHz (and integer multiples thereof).

FIG. 4 shows flow chart of a method for changing clock configuration.This method is performed after the method of FIG. 3 determines theclocking configuration to be used in a measurement. Step 180 is checkingif the current clocking configuration matches that selected by themethod of FIG. 3. If the current clocking configuration matches theselected clocking configuration, then this method terminates. If not,Step 182 is performed. Step 182 has the microcontroller 33 select aninternal RC time base. Step 184 has the microcontroller 33 send a clockchange command to the master clock synthesizer 29. Step 186 has themicrocontroller select the clocking from the master clock signal throughclock lines 47. This step is performed after the master clocksynthesizer 29 has stabilized in its new configuration.

The advantages of this device and method include, without limitation,very low production cost, very light weight, reduced need for innershielding, very good performance specifications in narrow resolutions,very good rejection of residual, spurious and image signals generatedfrom the low-cost part selection, and accurate, repeatable measurementsby connecting this device directly to the signal being measured withoutneed for additional cables, similar to an RF power sensor but with fullspectrum analysis and measuring receiver capabilities.

Those skilled in the art will recognize that numerous modifications andchanges may be made to the preferred embodiment without departing fromthe scope of the claimed invention. It will, of course, be understoodthat modifications of the invention, in its various aspects, will beapparent to those skilled in the art, some being apparent only afterstudy, others being matters of routine mechanical, chemical andelectronic design. No single feature, function or property of thepreferred embodiment is essential. Other embodiments are possible, theirspecific designs depending upon the particular application. As such, thescope of the invention should not be limited by the particularembodiments herein described but should be defined only by the appendedclaims and equivalents thereof.

What is claimed is:
 1. A method comprising the steps of: sequentiallyconverting, using one or more mixers, a radio frequency signal to asequence of intermediate frequency signals using a sequence of localoscillator signals generated by a local oscillator, each of the localoscillator signals having a different frequency; sequentiallyconverting, with an analog-to-digital converter, the sequence ofintermediate frequency signals to a sequence of digitized intermediatefrequency signal sample sets; converting the sequence of digitizedintermediate frequency signal sample sets to a sequence of frequencydomain sample sets, each frequency domain sample set comprising afrequency value and signal energy value; dividing a frequency span intoa plurality of frequency sample bins; associating, for each frequencysample bin, all frequency domain samples from the sequence of frequencydomain sample sets having a frequency value within the frequency samplebin; and selecting for a combined frequency domain sample set, for eachfrequency sample bin, out of the frequency domain samples associatedwith the frequency sample bin, the frequency domain sample having thesignal energy value that is lowest.
 2. A method comprising the steps of:converting a sequence of digitized intermediate frequency signal samplesets to a sequence of frequency domain sample sets, each frequencydomain sample set comprising a frequency value and signal energy value;dividing a frequency span into a plurality of frequency sample bins;associating, for each frequency sample bin, all frequency domain samplesfrom the sequence of frequency domain sample sets having a frequencyvalue within the frequency sample bin; and selecting for a combinedfrequency domain sample set, for each frequency sample bin, out of thefrequency domain samples associated with the frequency sample bin, thefrequency domain sample having the signal energy value that is lowest.3. The method of claim 2, further comprising the steps of: sequentiallyconverting, using one or more mixers, a radio frequency signal to asequence of intermediate frequency signals using a sequence of localoscillator signals generated by a local oscillator, each of oscillatorsignals having a different frequency; and sequentially converting, withan analog-to-digital converter, the sequence of intermediate frequencysignals respectively to the sequence of digitized intermediate frequencysignal sample sets.
 4. A non-transient computer readable medium encodedwith instructions that when executed by a processor cause the processorto perform the steps of: converting a sequence of digitized intermediatefrequency signal sample sets to a sequence of frequency domain samplesets, each frequency domain sample set comprising a frequency value andsignal energy value; dividing a frequency span into a plurality offrequency sample bins; associating, for each frequency sample bin, allfrequency domain samples from the sequence of frequency domain samplesets having a frequency value within the frequency sample bin; andselecting for a combined frequency domain sample set, for each frequencysample bin, out of the frequency domain samples associated with thefrequency sample bin, the frequency domain sample having the signalenergy value that is lowest.